Civil Engineering/Construction joint in RCC sunshade

Presently we are constructing a 02 storey building of size 44m X 10m. All around the building, a continuous RCC sloping sunshade has been planned. The RCC sunshade will be emerging from the bottom of the RCC roof beam ( 10mm dia Fe500 steel @ 150mm c/c)

While doing the ground floor roof concreting, we thought to do the sunshade later and we left the dowel bars in the roof beam .i.e 10mm dia Fe500 steel @ 150mm c/c. We have also plans to apply concrete bonding agents in the market for bonding old and new concrete while doing the sunshade later. However, the joint will be in junction of beam and sunshade throughout.

Whether what we have done are right ? we are now going to make the first floor slab also and plan to follow the same. Please advise us whether monolithic casting of roof beam and sunshade are essential ? whether we could go ahead with a concrete joint at beam/sunshade junction with the help of bonding chemicals in market?

regards,

Geoffrey,
India.

AnswerHi Babal,

Yes, what you have proposed is fine. Just ensure that before you cast the projecting sun-shade, you scabble the surface of the old concrete to expose the aggregate- after that you can apply the bonding agent and cast the new concrete.

An alternative and even better method is to bend the canopy bars at right angles, running along the outside cover of the 1st floor beam, and enclose these bars in polystyrene to a depth (thickness) of say 15-20mm (the bars and the polystyrene will be adjacent to the shuttering of the beam, outside the main beam rebar/links). The height of the polystyrene insert will be the same as the thickness of the canopy. Then, when you remove the side shutters of the beam, and after say 14 days, you can chisel out the polystyrene and straighten out the rebar and you will get a 20mm rebate within the beam, which will form an excellent shear key for the cantilevering canopy.

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I can answer any questions to do with civil and structural engineering consultancy and construction industry in East Africa and the Middle East, and specifically with the analysis and design of reinforced concrete structures. My particular expertise is in the aseismic design and optimisation of tall buildings.